The response of photographic materials to exposure is usually defined by a sensitometric or characteristic curve or set of curves. Such a curve plots image density in the processed material versus the log exposure required to provide that density. The sensitivity, or speed, of a photographic material is a function of the minimum exposure required to provide a detectable change in density compared to density observed in unexposed areas of the material. This is referred to as the threshold speed. In negative photographic materials, the threshold speed is the minimum exposure level required to provide a detectable increase in density compared to the background density observed in unexposed areas of the material. In reversal materials, the threshold speed is the minimum exposure required to provided a detectable decrease in density compared to the maximum density observed in unexposed areas of the material.
The image density of a photographic material can be provided either by metallic silver or by a dye. Coupler compounds are widely used in photographic materials to form dye images by reacting with the oxidation product of silver halide developing agents. Pyrazoloazole coupler compounds are well-known as magenta dye-forming couplers. Such couplers are described in, for example, U.S. Pat. Nos. 4,443,536, 4,665,015, 4,639,415, 4,639,413, 4,559,297, 4,618,573, and 4,762,775.
While pyrazoloazole compounds are useful dye-forming couplers, photographic materials and compositions utilizing them often do not provide as high a threshold speed as might be desired. It is thus an object of this invention to provide higher threshold speed for photographic compositions containing pyrazoloazole coupler compounds.